Evapotranspiration And Energy Balance In A Farmland Ecosystem On The Loess Tableland

Evapotranspiration and energy balance are the important components of farmland ecosystem processes and functions. Winter wheat ecosystem on the Loess Tableland was taken as the study object. Eddy covariance and meteorological data were used to analyze the dynamic variation of evapotranspiration, energy closure and distribution characteristics,which are important for illustrating the regional characteristics of water-heat balance and improving the water use efficiency of farmland. The main conclusions are as follows:(1) In the typical months of the winter wheat field, i.e., from August 2008 to July 2009,the diurnal variation of evapotranspiration showed a single peak curve. Evapotranspiration was mostly positive in the days, which showed that water vapor transported from winter wheat field to atmosphere. There was significant difference between diurnal variation peaks of every month. The maximum value appeared in May and the minimum value, in January. In general, the peak value of daily evapotranspiration appeared at noon. There was significant difference between evapotranspiration amounts under different weather conditions(sunny and cloudy). Under the influence of environmental factors, evapotranspiration in fine days was obviously higher than that in cloudy days. Especially, the difference in spring was the largest,followed by summer, autumn and winter.(2) The annual evapotranspiration of the winter wheat field was 522.6 mm, which was20.8 mm higher than precipitation. The evapotranspiration was significantly less than precipitation in July and September and significantly higher than precipitation in April and June, which was mainly related to the growth of winter wheat. Evapotranspiration in the winter wheat field was largely restricted by environmental and biological factors.Environmental factors mainly included net radiation, air temperature and soil water content.Especially, evapotranspiration was significantly affected by net radiation. Biological factors mainly related to LAI.(3) The energy closure degrees were 0.71 at half hour scale and 0.76 at daily scale,respectively, which showed that energy was not closed in the winter wheat field. For the different time scales, energy closure degree in day time was obviously higher than that at night. It was the main reason for the decrease in data quality that energy was not closed at night; energy closure degree in summer was higher than that in winter; and energy closure innon-growing season was higher than that in growing season, because energy closure in the wintering period was low.(4) The temporal variation of energy balance components in the winter wheat field was mainly affected by meteorological conditions and the growth conditions of winter wheat.Annual energy was mainly distributed to latent heat flux, followed by sensible heat flux and soil heat flux. Sensible heat flux was the dominate item in the wintering period. In grain filling period, latent heat flux and sensible heat flux were roughly equal. Latent heat flux was the dominate item in other growing periods and fallow period.